1. Field of the Invention
The present invention relates to a magneto-optical recording apparatus for repeatedly overwriting information on a magneto-optical recording medium with an intensity-modulated laser beam.
2. Description of the Prior Art
Magneto-optical recording/reproducing apparatus for reading information bits, i.e., magnetic domains, through magneto-optical interaction employ different recording processes for rewriting recorded information or performing a repeated overwrite capability. These recording processes include a magnetic-field-modulated recording process which modulates an external field to be applied to a magneto-optical recording medium and a double-head recording process which uses a recording head and an erasing head.
One magnetic-field-modulated recording process is disclosed in Japanese laid-open patent publication No. 60-48806, for example. According to the disclosed process, to record information on a recording medium with an amorphous ferrimagnetic thin film having an easily magnetizable axis extending perpendicularly thereto, a laser beam is applied to an overwrite region on the recording medium to heat the same, and a magnetic field with its polarity reversed according to the information to be recorded, e.g., a value of "0" or "1", is perpendicularly applied to the ferrimagnetic thin film to magnetize the same for thereby recording the information. However, attempts to record desired information on the recording medium at high speed with a high information transfer rate require an electromagnet that can operate at high frequency. Such an electromagnet cannot easily be fabricated, consumes a large amount of electric energy, and dissipates a large amount of heat.
According to the double-head recording process, any recorded information on the recording medium is erased, or "initialized", by the erasing head before desired information is overwritten. Since the two heads are positioned in spaced-apart relationship, the magneto-optical drive systems based on the double-head recording process require a complex manufacturing and assembling process, are relatively large in size, and hence are not economic.
To solve the above problems, there have been proposed thermomagnetic (magneto-optical) recording methods as disclosed in Japanese laid-open patent publications Nos. 63-52354, 63-268103, and 2-24801.
As shown in FIG. 1 of the accompanying drawings, the recording medium disclosed in Japanese laid-open patent publication No. 63-52354 employs a magnetic recording medium 20 composed of a memory layer 1 and a recording layer 2 which are magnetically coupled to each other and disposed on a substrate 11, each of the memory and recording layers 1, 2 comprising a perpendicularly magnetizable magnetic thin film of amorphous-rare-earth transition-metal alloy. The memory and recording layers 1, 2 have respective coercive forces H.sub.Cl, H.sub.C2 (H.sub.Cl &gt;&gt;H.sub.C2) and respective Curie temperatures T.sub.C1, T.sub.C2 (T.sub.C1 &lt;T.sub.C2).
While an external magnetic field Hex is being applied to the recording medium 20, the recording medium 20 is switched, depending on the information to be recorded, e.g., a value of "0" or "1", between a first heated state in which the recording medium 20 is heated to a first temperature T.sub.1 that is higher than the Curie temperature T.sub.Cl of the memory layer 1 and at which the sublattice magnetization of the recording layer 2 is not reversed, and a second heated state in which the recording medium 20 is heated to a second temperature T.sub.2 that is higher than the Curie temperature T.sub.Cl and which is high enough to reverse the sublattice magnetization of the recording layer 2. When the recording medium 20 is cooled, the orientation of the sublattice magnetization of the memory layer 1 is aligned with that of the recording layer 2 due to the exchange force between the memory and recording layers 1, 2, thus forming a recording bit, i.e., a magnetic domain, of "0" or "1", in the memory layer 1 and also reversing the sublattice magnetization of the recording layer 2 into a direction (hereinafter referred to as a normal direction) at the time the recording medium 2 was initialized, under an initializing external magnetic field H.sub.i applied to the recording layer 2. In this manner, the recording medium 2 is brought into a condition in which desired information can be overwritten on the recording medium 2.
The above recording medium does not need any special process or time for erasing recorded information, and allows desired information to be recorded at high transfer rate. The disclosed arrangement is also effective to solve the problems with the double-head recording process and the magnetic-field-modulated recording process.
The recording medium disclosed in Japanese laid-open patent publication No. 2-24801 employs a magnetic recording medium 20 as shown in FIG. 2 of the accompanying drawings. The recording medium 20 comprises the substrate 11, the memory layer 1, and the recording layer 2 which are identical to those shown in FIG. 1. The recording medium 20 additionally has an intermediate layer 3 disposed between the memory and recording layers 1, 2 for controlling the magnetic wall energy therebetween for reliable reversal of the sublattice magnetization of the recording layer 2 into the normal direction.
To record desired information on the recording medium 20 shown in FIG. 2, the recording medium 20 which may typically in the form of a disk, is rotated by a drive motor 25. A recording unit for recording the information comprises an optical head for applying a laser beam for heating the recording medium 20 selectively to the first and second temperatures T.sub.1, T.sub.2, and a recording magnetic field generator such as a permanent magnet for applying an external magnetic field H.sub.ex to the recording medium 20. An initializing magnetic field generator such as a permanent magnet is provided, downstream of the recording magnetic field generator in the direction in which the recording medium 20 rotates, for applying an initializing magnetic field H.sub.i to initialize the recording layer 2.
The initializing magnetic field H.sub.i has an intensity smaller than the coercive force H.sub.Cl of the memory layer 1. In order to reliably direct the sublattice magnetization of the recording layer 2 into the normal direction which is perpendicular to the recording layer 2, the initializing magnetic field generator has its N and S poles oriented along a direction perpendicular to the plane of the recording medium 20 for applying a perpendicular magnetic field whose intensity is greater than at least the sum of the coercive force H.sub.C2 of the recording layer 2 and the exchange force H.sub.W2. In addition, the permanent magnet as the initializing magnetic field generator is selected to generate a relatively large magnetic field intensity large enough to produce the necessary perpendicular magnetic field, or an auxiliary magnetic pole or magnet of high magnetic permeability is disposed opposite to the initializing magnetic field generator with the recording medium 20 interposed therebetween, for applying an intensive perpendicular magnetic field efficiently to the recording medium 20.
However, since the auxiliary magnetic pole is disposed in confronting relationship to the magneto-optical recording medium, the overall magneto-optical recording apparatus is relatively large in size, complex in structure, and cannot easily be assembled. Nevertheless, increasing the perpendicular magnetic field without resorting to the auxiliary magnetic pole would cause various problems such as leakage flux and the need to increase the capability of the magnetic field generator.